Despite the incredible tragedy of Sept. 11, 2001, and numerous attacks in the years since, many Americans continue to underestimate the potential danger posed by radicalized Muslims within the U.S.

According to one of the two men responsible for killing British soldier Lee Rigby earlier this year, one such American was integral in his decision to carry the vicious attack out. Michael Adebowale, who reportedly hit the victim with his vehicle before fatally cutting him with a meat cleaver, was recently convicted of Rigby’s death.

In a subsequent letter, Adebowale cited the influence of Khalid Yasin, an American Muslim leader, as the impetus behind the unprovoked attack.

Yasin’s message was integral in Adebowale’s 2009 conversion to Islam, the jihadist claimed in his letter. After hearing one of the American’s lectures, the convicted murderer began moving closer to Islam.

“After hearing this I had to conclude that the purpose of life is to acknowledge and worship the creator,” he wrote.

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He went much further than worship, however, when he mercilessly slaughtered a man in public and continued to portray the act as his religious duty. Yasin’s teachings have a similarly murderous tone, which seems to have played a significant role in Adebowale’s development into a radical Muslim.

Now 67, Yasin has been a practitioner of Islam for about four decades. The former gang member has expressed his desire to see homosexuals murdered and claims the U.S. government invented AIDS. He also teaches that higher education is an abomination of the faith.

Obviously, it is easier to convince an uneducated individual that jihad is an appropriate response to the world’s non-Muslim population.

While Adebowale and his cohort, Michael Adebolajo, were found guilty of an unconscionable murder, they will likely spend the rest of their lives behind bars. The residual travesty is that those who incite such violence remain free to spread their hate.